It is common practice in electric arc furnaces, and in other applications in steel and metallurgical industries, to inject technological gases and solid material in powder form above and inside the bath of melting metal.
Generally speaking, technological gases should be taken to mean oxygen, nitrogen, argon, methane, propane, air or other gases with like characteristics.
Generally speaking, solid material in powder form should be taken to mean the powders of coke, carbon, iron alloys, lime, dolomite or other materials with like characteristics.
There are many purposes for this injection, among the most important being:
to provide an energy input for melting; PA1 to activate the oxidation step and influence the dephosphorization and the desulphurization of the liquid metal; PA1 to encourage slag foaming; PA1 to facilitate the melting and shearing of the scrap; PA1 to encourage the stirring of the bath, thus accelerating the time taken to activate the chemical reactions; PA1 to actuate decarburation, that is to say, the regulation of the carbon content, and to control the tapping temperature; PA1 to obtain the burner functioning mode, wherein the oxygen or air enriched with oxygen act as comburents of natural gas, methane, oil, propane, butane, carbon or other solid or gassy fuels. PA1 a) by means of water-cooled lances equipped at the ends with a nozzle, for example of the convergent/divergent type, suitable to produce a supersonic jet at outlet; PA1 b) by using lances of the consumable type. PA1 the need for manipulation; PA1 the gassy jet loses energy due to the distance from the bath, which often implies it is impossible for the jet to penetrate inside the bath; PA1 large quantities of cooling water are needed to prevent the tip of the lance from being destroyed due to the heat and the mechanical stresses; PA1 danger of explosions caused by possible water leakages; PA1 if the lance is very close to the bath there is a risk of damage due to the heat, the tip may be washed, encrustations of steel may form and there is a risk of erosion; PA1 if the lance is very close to the scrap, the gassy jet may be deflected and even reflected against the end or the side of the lance and therefore cause damage thereto; PA1 another lance is needed for the combustion of the CO which escapes from the bath; PA1 other devices are needed to inject the solid material in powder form. PA1 high cost of the parts which are consumed; PA1 it is difficult to determine the exact positioning of the tip of the lance; PA1 the method is not very efficient at distributing the point of impact of the jet; PA1 due to overheating the lance may bend; PA1 it is necessary to add new segments of lance as it is gradually consumed; this requires a wide use of equipment and manipulators which are costly and bulky; PA1 further devices are needed for the post-combustion and injection of the solid material in powder form. PA1 reduction of tap-to-tap time; PA1 reduction of electric energy consumed; PA1 reduction of electrode consumption; PA1 improved penetration of the gassy jet into the bath of metal; PA1 more accentuated turbulence in the bath, which entails a more uniform temperature and a quicker melting of the scrap; PA1 greater melting intensity; PA1 greater productivity and greater efficiency in the use of the oxygen in the bath; PA1 reduced concentration of oxygen in the liquid bath and therefore better quality of steel; PA1 when coupled with electromagnetic stirrers, in some cases it allows to eliminate the function of the bottom tuyeres; PA1 reduction of erosion of the refractory; PA1 greater efficiency in post-combustion and reduction of the carbon oxide in the gases discharged from the furnace; PA1 reduction of the water cooling of the injection means; PA1 more efficient use of foamy slag technique.
In the state of the art two main solutions are adopted to inject oxygen or other gases inside a liquid bath:
Using water-cooled lances entails the following disadvantages:
The disadvantages of consumable lances are as follows:
Of the two solutions, in recent years the use of supersonic lances has particularly developed; with these it is possible to inject the necessary quantity of oxygen by means of a jet with a speed higher than that of the sonic speed of the fluid in the relevant conditions of supply temperature and pressure.
However, with present-day technology, the oxygen is not injected an optimum manner of functional to the melting process.
In fact, in systems known to the stat of the art, the impulse of the jet of oxygen is insufficient to penetrate the bath of liquid metal to a depth sufficient to ensure that the oxygen is adequately distributed throughout the bath (for example equal to half the overall height of the bath).
At the moment of impact with the surface of the bath, the jet generates impact waves of compression of very high intensity, which cause a dissipation of the jet and a dispersion of the gas on the surface of the bath, so that only a minimum part of the gas penetrates into the liquid bath of molten metal.
This impact against the surface of the bath also causes a loss of coherence and parallelism in the fluid threads in the jet, with a resultant loss in its penetrative ability.
Moreover, the system makes it necessary to mount the supersonic lance on a manipulator, or another mechanical organ which allows the lance to be moved, in order to adjust the outlet distance with respect to the surface of the bath, since the jet of oxygen tends to disperse after a few centimeters, in the order of a few dozen cm, from the outlet of the supersonic nozzle.
For this reason, consolidated practice provides to insert the end part of the supersonic lance inside the layer of slag above the bath to ensure that the oxygen is introduced inside the liquid bath in a sufficiently efficient manner, but in any case this is not an optimum solution.
EP-A-874.194, which discloses the pre-characterizing part of claim 1, describes a burner which can be used on electric arc furnaces comprising a first, inner nozzle with a convergent-divergent development (Laval nozzle) which emits a mixture of oxygen and natural gas, and a second nozzle, coaxial to and outside the first, which emits particulate material.
In this document, the purpose is substantially to allow the flow of particulate material to mix with the primary flow of oxygen and fuel, so that the material can be distributed uniformly in the flame produced by the burner and can be projected as far as possible inside the furnace.
The outer nozzle defines a straight flow path for the particulate material in order to prevent abrasions on the wall due to the passage of said material.
In this document, the flow delivered by the outer nozzle does not form a protective crown for the primary flow delivered by the inner nozzle, but mixes immediately therewith, already inside the burner itself, since it is drawn by the high increase in pressure created by the supersonic acceleration of the oxygen and fuel.
Moreover, this document does not provide variable working options to modify the composition and the development of the flame according to the various steps of the melting process, so that the regulation of the working of the burner is not correlated to the development of the melting cycle and to the different technological requirements which gradually occur.
The present Applicant has designed, tested and embodied this invention to overcome all these shortcomings and to obtain further advantages.